I see this as a necessity mainly for server software which you don't want to change too often. Ubuntu does a great job at this with their LTS releases.
But on the other hand with the way the android adoption curve looks, that's still a lot of people. I just checked wikipedia right now, something like 35% of users are running a version which google stopped patching. So to have the same coverage of security updates, google has to support more OS versions. And even then, they're leaving a lot of people in the dust.
I'm sure google would like people to upgrade too, less versions for them to maintain, but they've got the whole carrier situation to deal with in that ecosystem.
But in the consumer mobile OS space, there are really only a few big players. It looks like google does patch old versions of the OS, albeit up to a limit: http://blogs.wsj.com/digits/2015/01/12/google-not-fixing-som...
But on the other hand with the way the android adoption curve looks, that's still a lot of people. I just checked wikipedia right now, something like 35% of users are running a version which google stopped patching. So to have the same coverage of security updates, google has to support more OS versions. And even then, they're leaving a lot of people in the dust.
I'm sure google would like people to upgrade too, less versions for them to maintain, but they've got the whole carrier situation to deal with in that ecosystem.